(1) Field of the Invention
The present invention relates to a substantially linear, high-molecular-weight soluble novolak resin and a process for the preparation thereof.
(2) Description of the Related Art
A phenolic resin obtained by curing and molding a soluble novolak type phenolic resin or a cresol type phenolic resin has long been known, and since this resin has such excellent heat resistance, infusibility, and flame retardancy, it cannot be replaced by other resins. Therefore, even now, this resin is very important and used in many fields.
The soluble phenolic resin, especially a novolak type phenolic resin obtained in the presence of an acidic catalyst (hereinafter referred to as "novolak resin"), can be easily modified by the introduction of various reactive groups. Further, the novolak resin is alkali-soluble and the content of the aromatic ring is high, and therefore, the novolak resin has an excellent heat resistance, and a plasma etching resistance. Moreover, because of the carbon content of the novolak resin is high, many characteristic carbonized materials can be formed. Because of these many excellent characteristics, this novolak resin is in demand as a photoresist for IC, a sealing material, a matrix resin for a composite material, or a new material for a carbon fiber or an active carbon fiber, and is regarded as an important material in many fields.
Although the conventional novolak resin has such excellent characteristics, it has disadvantages in that the softening point is low and the mechanical strength is unsatisfactory. Various causes of these disadvantages have been considered, but the largest cause is deemed to be a low molecular weight.
The upper limit of the number average molecular weight of the conventional novolak resin is regarded as being about 1,000. When the molecular weight is below 1,000, the melting point rises in accordance with the increase in the molecular weight, and within this range, the softening temperature depends greatly on the molecular weight. The same things can be said about other properties, and thus indicates that it is difficult to prepare a novolak resin having certain properties with a good reproducibility. One of the objects of the present invention is to prepare a high-molecular-weight novolak resin having certain properties with a good reproducibility.
Heretofore, various trials have been made to prepare high-molecular-weight novolak resins from phenols and aldehydes, especially formaldehyde or a derivative thereof, in the presence of an acidic catalyst. However, as pointed out in the thesis of Drumm [M. F. Drumm and J. R. LeBlanc, Kinet. Mech. Polym., 1972 (3), 157], it is considered difficult to obtain a novolak resin having a number average molecular weight exceeding 1,000.
J. Borrajo et al. theorized that if the reaction is carried out at a molar ratio of formaldehyde (F) to phenol (P) (which molar ratio is hereinafter referred to as "F/P molar ratio") adjusted to 0.881, when the formaldehyde is completely reacted, gelation is caused, and to prevent this gelation, i.e., insolubilization, the F/P molar ratio must be kept below 0.88 [J. Borrajo et al., Polymer, 23, 263 (1982)].
Japanese Unexamined Patent Publication (Kokai) No. 58-32,622 proposed a process in which a mixture of a phenol and a carbonyl compound is heated in the presence of an acidic catalyst to be thereby reacted to form a novolak resin having a high number average molecular weight and a very broad molecular weight distribution. Also in this process, to prevent gelation, for example, in the case of phenol and formaldehyde, the F/P molar ratio must be kept below 0.833.
As is well-known, to obtain a polymer having a high molecular weight, the F/P molar ratio must be 1.00, and at the F/P molar ratio of 0.833 as taught in Japanese Unexamined Patent Publication No. 58-32,622, a high molecular weight cannot be attained. If the molecular weight distribution of the novolak resin obtained under the above conditions is extremely broad, it is considered that the obtained resin is highly branched and extremely unstable.
It is admitted that even if a bifunctional phenol having no risk of insolubilization, such as o-cresol, p-cresol, o-chlorophenol or p-chlorophenol is used, it is difficult to obtain a phenolic resin having a high molecular weight. Japanese Unexamined Patent Publication No. 59-191,710 teaches that a novolak resin having a molecular weight of several thousands can be obtained only when special means is adopted. Nevertheless, it is taught that, even under such conditions, a gelatinous polymer is formed if the content of the bifunctional phenol is lower than 70 mole %, and a substantially linear novolak resin having a high molecular weight cannot be obtained.
Although various trials have been made to obtain novolak resins having a high molecular weight, as pointed out above, a practical technique capable of producing a novolak resin comprising a polyfunctional phenol as the main component, which has a high molecular weight and is soluble, for industrial utilization in a high yield economically advantageously, has not been discovered.
We carried out research with a view to increasing the molecular weight of a novolak resin without degrading the excellent characteristic properties of the novolak resin, and as a result, we found a novolak resin having a heretofore unexpected high molecular weight and a high softening point, in which the dependency of the softening point on the molecular weight is reduced, and thus have completed the present invention.